In general, a via is a hole within a circuit board which is lined with conductive material (e.g., copper, nickel, etc.) to provide an electrical pathway between multiple conductive layers of the circuit board. A typical circuit board includes many vias in order to achieve layer-to-layer interconnections between various conductive circuit board features (e.g., surface pads and traces, inner functional signal layers, power planes, ground planes, etc.).
One type of via, which extends completely through a circuit board (i.e., from one outer surface to the other outer surface on the opposite side of the circuit board), is called a “through via”. Some circuit board manufacturers do not draw distinctions between the term “through via” and the term “plated through hole” (PTH), which refers to a structure that is intended to support a component lead (e.g., a pin in hole configuration). Accordingly, within this document, the term “through via” is intended to refer generally to both through vias and plated through holes.
Another type of via, which extends only partially through a circuit board (i.e., the via hole does not pass completely through the circuit board), is called a “blind via”. A blind via typically starts on one outer surface of the circuit board and extends perpendicularly into the circuit board to the first inner signal layer or plane. Traces and planes beneath this layer are allowed to pass under the blind via without interruption thus enabling the manufacturer to locate a higher density of conductive circuit board features in the same area (e.g., blind vias on both sides of the circuit board, inner signal layers and conductive planes underneath the blind vias, etc.).
In general there are two conventional approaches to creating blind vias. One approach involves creating through vias in two thinner circuit board sections and then combining these sections together (e.g., using a laminating and pressing process) to form the circuit board. Another approach involves drilling out blind via holes from a circuit board substrate, and then forming both the blind vias and through vias concurrently using a plating process. In this latter approach, the aspect ratio between via hole depth and via hole width is substantially limited in order to guarantee reliable cleaning and plating and to avoid trapping process chemistries (e.g., acid traps). For instance, in order to comply with a design rule imposing a blind via aspect ratio which is no greater than 0.5-to-1 to enable reliable plating and cleaning, a blind via which is 8 mils in diameter (i.e., 0.008 inches) is allowed to extend up to 4 mils deep to the next conductive layer of the circuit board.
Another type of via, which is completely embedded within a circuit board (i.e., the via hole does not extend to any outer surface of the circuit board), is called a “buried via”. A buried via connects two or more inner conductive layers but is not directly visible on the outer surface of either side of the circuit board. Traces and/or conductive planes above and below the buried via are allowed to pass without interruption thus enabling the manufacturer another opportunity to locate a higher density of conductive circuit board features in the same area (e.g., signal traces, conductive planes, etc. above and below the buried via). Although buried vias are not directly accessible since they do not extend to an outer circuit board surface, other vias (e.g., through vias and/or blind vias) typically connect to buried vias by way of inner layer signal traces to provide external electrical access to the buried vias.